The complex bacterial masses which develop on and adhere firmly to teeth, collectively known as dental plaque, provide most of the causes of dental caries and periodontal diseases. In the adherence of plaque, bacterial polysaccharides are important but we are only beginning to understand the variety of molecules, their nature and their interactions which are responsible for this phenomenon. A thorough understanding of these molecular interactions should lead to innovations for the removal of plaque or the suppression of its development, and thereby should contribute importantly to the control of dental caries and periodontal diseases. Among the important interactions are numerous examples of specific coaggregation between bacteria of different species in plaque, e.g., Streptococcus sanguis and Actinomyces viscosus. S.sanguis is a very common, and often the most abundant bacterium identified in the early colonization of freshly cleaned teeth in man. Actinomyces, which are implicated in root caries and periodontal diseases, increase in number rapidly during the first several days of plaque formation. The coaggregation between these two may be important in the build-up of the Actinomyces population. The objective of this proposal is to provide an understanding of the molecular basis of coaggregation between S.sanguis 34 and A. viscosus T-14. To achieve this goal we plan to do the following: (1) Determine the conditions under which the coaggregation is reproducible and can be studied quantitatively e.g., pH, specific ionic requirements, bacterial culture conditions, etc.; (2) Determine the nature of the molecules on the surface of each bacterial species which are necessary for the coaggregation; (3) Isolate the coaggregation molecules in highly pure form and determine as much as is feasible of the structural features of the specific reactive sites.